1. Field of the Invention
The present invention relates to electronic power supplies and specifically to methods of measuring and limiting current delivered to a load.
2. Description of the Prior Art
Two types of power supplies are in wide use that convert an incoming source to voltage levels useful to the equipment being powered. Linear power supplies are the simplest, a series transistor typically drops enough voltage across it to maintain a constant output voltage. The problem with linear power supplies is that the heavier the output load, the more current will have to pass through the regulating transistor, since it is in series with the load. This increased current means more power must be dissipated by the transistor, and this power is thrown away as heat. Switching power supplies are more efficient and run cooler than linear power supplies at the same power levels. But switching power supplies are more complicated because a transistor operated as a switch is used to chop incoming current for a transformer that has the load connected to its secondary winding, usually through a rectifier and filter. A feedback circuit is needed to monitor the output voltage and increase the time duration the chopping transistor is on when more output voltage is needed and to reduce the time duration when less output voltage is needed. The feedback circuit often makes use of a third winding of the transformer. The advantages of the switching power supply are that they can operate either as step-up or step-down supplies, by adjusting the number of series conductors in the primary and secondary windings of the transformer, and less heat is dissipated by the transistor since it never operates in its linear region. The transistor is either fully off or fully on.
A problem develops in power supplies that use current limiting. A conventional method of current limiting in a pulse width modulation (PWM) type switching power supply is shown in FIG. 1. A power supply 10 has a resistor (Rs) 12 in series with a power switch transistor 16 and a load 18. A sense voltage (Vs) develops across Rs 12 that is the product of the current and the resistance (Ohm's Law). This voltage is sensed by an input of an comparator 20 and compared to a reference voltage (Vref). A logic block 22 accepts the comparison from comparator 20 and controls the gate of transistor 16. As Vs exceeds Vref, transistor 16 will turn off. When Vs drops below Vref, transistor 16 will turn on. The total effect is to limit the current through transistor 16, and therefore load 18, to a value that can be adjusted by Vref. The disadvantages of putting Rs 12 where it is, include reduced gate drive at higher currents because the Vt of transistor 16 increases and the source lifts from ground as more Vs develops. To combat this, the value of Rs 12 is kept very low, on the order of 0.1 ohms. But at such low resistance values, the trace resistance on a printed circuit board can become significant, and can ruin any precision. An accurate reference source is needed to produce Vref. And low values of Vs, which can typically be 200 millivolts, do not compete with noise very well. The current mode control which requires dynamic adjustment of the current limit based on several control inputs is difficult to implement.
In FIG. 2, a switching power supply control circuit 30 puts a sense resistor (Rs) 32 "below ground." A current (Is) through Rs 32 also passes through a switch transistor 34 and a load 36. A negative sense voltage (-Vs) develops across Rs 32 as a result of Is because circuit or chip ground is at the junction of Rs 32 and transistor 34. An comparator 38 must have a negative reference voltage (-Vref) so that it can compare -Vs. Although the problem of transistor source terminal lifting has been avoided, the negative -Vref is troublesome to derive. Only small threshold voltages can be used (e.g., 150-200 millivolts) if power supply 30 is implemented mostly on a chip, larger thresholds will forward bias the substrate of the chip.
What is needed is a method and apparatus for accurate current limiting without the problems inherent in the prior art.